I first heard about the impact of cows' farts some years ago, so it's not a new theory - if you add into it the farts of all ruminants, like the millions of wildebeeste, bison, moose, elk, reindeer, etc., roaming the planet and blowing-off constantly, not to mention the millions of equines worldwide that have to let one off every now and then, that's a lotta, lotta methane. And there are millions more people in the world now - even over the last 100 years, we've increased longevity, so people are farting longer.
There's a thought to conjure with - the Blue Planet rendered dead as a doornail in 1000 years' time (instead of 5 billion, when the Sun dies), because of global flatulence. Just like that: pfffftttttparp...